Thank you so much for your help thus far.

The problem is that these duplicate entries do not appear in the Categorised section of the Banking page, only the actual deposits do (for the amount minus the Zettle fees). 

The duplicated (total amount) entries appear on the Bank Account History page, thereby artificially inflating the balance.  They also appear on the Invoices page (marked Deposited). It seems Zettle has been creating an invoice for these deposits (and immediately marking it as paid) in addition to (correctly) entering it on the banking page.
I suspect this may be due to incorrect field mapping in the Zettle Integration whereby we had allocated TSB Business Bank Account as the Zettle liquid account in the Zettle Payments section (see screenshot).

I am awaiting a reply from Zettle Support regarding the correct settings moving forward, but meanwhile, how can I remove the duplicated entries from the Bank Account History page? Can I simply delete them? Will deleting them from the Invoices page do the job?